It is well known that lasers can be used with great efficacy in a wide variety of applications. For each particular application, this efficacy is significantly dependent on the physical characteristics of the generated laser beam. When a laser beam is pulsed, the principle variables include the wavelength of the light in the beam, the time duration for each of the laser pulses, the repetition rate of the pulses, and the intensity of the laser during each pulse (i.e. the number of photons striking a target area in a given unit of time). It happens that a beam which can deliver laser energy in pulses is very efficacious for certain applications. This is particularly so when the laser pulses in the beam are of extremely short duration, e.g. several picoseconds. While no one variable can be considered in isolation, indeed all of the above-mentioned variables are important in shaping a pulsed laser beam, the concern of the present invention centers on factors affecting the ability of a regenerative amplifier to effectively control the intensity and repetition rate of pulses in the beam.
If the particular application is best accomplished using an uninterrupted continuous beam of light, then the duration of the beam is of minimal importance. On the other hand, it is known that many applications are best accomplished using a pulsed laser beam in which the intensity of each pulse and the repetition rate of these pulses can be of extreme importance. For example, several procedures in ophthalmic surgery require very intense light. If such light is not properly controlled, i.e. its intensity is not properly limited, the laser beam will invariably cause unnecessary damage to peripheral tissue. Consequently, the intensity of each pulse in the beam and the period of time between each of these pulses can be critical.
In a typical pulsed laser beam generating system, the intensity and duration of pulses in the beam are established by a regenerative amplifier. For this purpose, the regenerative amplifier includes a laser cavity wherein laser pulses are captured and reflected back and forth along a predetermined path. During the excursions of the laser pulses within the cavity, the pulses pass through a gain medium and are thereby amplified, to increase their intensity or energy level. To control the intensity of the light pulses in the resultant laser beam which emerges from the regenerative amplifier, and the repetition rate of the pulses in this beam, an electro-optical crystal is used which is of a type commonly referred to as a Pockel's cell.
The actual operation of an electro-optical crystal, and consequently the operation of a regenerative amplifier, is dependent on an important property of light which has not yet been mentioned, viz. polarization. Within a regenerative amplifier, it is important that the laser beam be polarized. This is so because, with an ability to selectively change the polarization of the laser beam while it is in the cavity of the regenerative amplifier, the beam can be selectively reflected back and forth in the cavity, or dumped out of the cavity. To do this, the beam must be properly polarized at a point where the laser is incident on a polarizing beam splitter. To obtain this proper polarization, an electro-optical crystal, i.e a Pockel's Cell, is effectively used as a switch.
Importantly, in its operation an electro-optical crystal relies on an electro-optic phenomenon whereby light passing through the crystal will be differently polarized depending upon whether the crystal is subject to an electrical field. Consequently, by selectively subjecting the electro-optical crystal to an electrical field, the polarization of light passing through the crystal can be controlled. This control over the polarization of the laser is obtained as a result of the electro-optic effect which occurs when an electric field is applied to an electro-optical crystal. In most cases, the electro-optic crystal also exhibits a piezoelectric effect in response to an electrical field whereby the electro-optical crystal is deformed by the electric field. Further, it is known that through the strain optic effect the deformation of an electro-optical crystal will also cause the crystal to change the polarization of light passing through the crystal. These various phenomenon need to be reconciled.
It happens that the electro-optic effect is effectively instantaneous. On the other hand, the piezoelectric effect occurs as a wave which is initiated at the surface of the crystal and which, if undamped, is reflected back and forth through the crystal. The consequent variations in the deformation of the electro-optical crystal causes the crystal to acoustically vibrate. Unfortunately, this unwanted vibration occurs both when the electrical field is applied to the crystal and when the electrical field is removed from the crystal. As mentioned above, if this vibration is undamped the consequence is that light passing through the crystal will be subject to rapid changes in polarization through the strain optic effect.
As might be expected, the "ringing" in electro-optical crystals is particularly pronounced when relatively high voltages are imposed on the crystal. The amount of rotation which the crystal can impose on light passing through the crystal, however, is a function of the applied voltage. Thus it happens that high voltages are required to effectively activate the electro-optical crystals which are used for switching the polarization of high intensity laser beams. Further, when high intensity laser beams are used for applications that require extremely precise procedures, such as ophthalmic surgery, the intensity and repetition rate of the pulses in the beam need to be established with precise certainty.
In a regenerative amplifier, the intensity of a laser pulse is increased or amplified by reflecting the pulse back and forth in a cavity to cause the pulse to repetitively pass through a gain medium. For effective amplification, however, the gain medium must be pumped to a level where it has sufficient energy which can be transferred to the pulse to thereby amplify the intensity of the pulse. Several time factors affect this process. First, there is the time necessary to pump the gain medium to a level where it has the requisite energy to amplify the pulse. Second, there is the time for amplification of the pulse. And third, there is the combined effect of the first and second factors which determines the time between pulses or the pulse repetition rate of the generated laser beam. It happens, that the "ringing" of the electro-optical switch influences all of these time factors. Specifically, consider the fact that a laser pulse will be trapped within the cavity of the regenerative amplifier while the electro-optical switch, i.e. the Pockel's Cell is electrically activated. During this time the pulse repetitively passes through the gain medium and is amplified. Then, upon deactivation of the electro-optical switch, the amplified pulse is released or "dumped" from the cavity. Once the switch is deactivated, the gain medium in the cavity must again be pumped to a higher energy level to amplify the intensity of the next pulse. Unfortunately, any ringing of the electro-optical switch retards the initiation of this process for pumping the gain medium. Thus, if the ringing of the electro-optical switch can be minimized, the effective pumping of the gain medium can be initiated earlier, and the pulse repetition rate of the generated laser beam can be increased. This repetition rate is, of course, a very important characteristic of a laser beam.
In light of the above, it is an object of the present invention to provide an optical switch for a laser cavity dumper which is effectively damped to reduce ringing by the switch. Another object of the present invention is to preserve an optical window through the crystal for a period of time during which this window, at the center of the crystal, experiences only the electro-optical effect caused by an applied electrical field and does not experience the "ringing" which is caused by acoustic waves. Yet another object of the present invention is to provide an optical switch which allows for a relatively high repetition rate for the generation of laser pulses. Another object of the present invention is to provide an optical switch for a laser cavity dumper which is able to provide relatively long optical windows (openings) for a regenerative amplifier. Still another an, object of the present invention is to provide an optical switch for a laser cavity dumper which is relatively easy to manufacture and comparatively cost-effective.